Method of making a hardened sheet metal article

ABSTRACT

In a method of making a hardened sheet metal article, a blank is formed with at least one through-passage with a projecting collar. Thereafter, the blank is heated and hot-formed in a press mold to a sheet metal article. Still within the press mold, the sheet metal article is hardened. The shaping of the blank in the press mold is accompanied by a calibration of the through-passage to final shape.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the priority of German Patent Application, Serial No. 101 49 220.0, filed Oct. 5, 2001, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method of making a hardened sheet metal article. In addition, the present invention relates to a press mold for carrying out the method.

[0003] To ensure clarity, it is necessary to establish the definition of several important terms and expressions that will be used throughout this disclosure. The term “blank” is used here in a generic sense and refers to a flat sheet steel plate as well as to a pre-formed semi-finished product. The term “heat-treating” or “heat treatment” is also used here in a generic sense and includes, i.a., quenching and tempering. The term “press mold” is also used here in a generic sense and includes a progressive die assembly which is capable to perform two or more operations, e.g. pressing, shaping, hardening etc.

[0004] German patent publication DE 24 52 486 A1 describes a method of making a hardened sheet metal article from a blank through a press hardening process by heating a blank of hardenable steel to a hardening temperature, placing the blank in a press mold for hot forming to the desired final shape, and hardening the final sheet metal product in the press mold. Since the final sheet metal product remains clamped in the press mold during the cooling process carried out during the hardening operation, the product is accuracy in size.

[0005] Through combination of the shaping and hardening and quenching steps in a single tool, hot forming and hardening in the press mold is an cost-effective process.

[0006] International patent publication WO 99/07492 discloses a modification of the afore-described press hardening process by collaring marginal areas of a plurality of holes in the press mold. The holes are collared in the press mold before the hardening process. The openings in the sheet metal product serve as through-bores for screw fasteners. These openings, also called in the art as “through-passages”, are utilized as reference holes or seats for accurate alignment of the sheet metal product in subsequent processes. In addition, these openings are also provided as assembly clearance or for reinforcement measure.

[0007] The components of this conventional press mold are subjected to great wear, when the through-passages are shaped. In addition, the collaring operation results only in a limited shaping of the collar. A further drawback is the increased costs compared to the conventional cold forming process.

[0008] It would therefore be desirable and advantageous to provide an improved method of making hardened sheet metal articles, which obviates prior art shortcomings and which can be carried out in a more economical and efficient manner.

SUMMARY OF THE INVENTION

[0009] According to one aspect of the present invention, a method of making a hardened sheet metal article, includes the steps of forming in a blank at least one through-passage with a projecting collar; hot-forming and hardening the blank in a press mold to a sheet metal article; and calibrating the through-passage to a final shape in the press mold.

[0010] The present invention resolves prior art problems by shaping the through-passage in the blank outside the heat-treatment process. As afore-stated, the blank may be a preform or a semi-finished product. Once the through-passage with its projecting collar has been made, the blank is heated and hot-formed in the press mold to the sheet metal article. At the same time, a calibration of the through-passage to its final shape is realized. Subsequently, the sheet metal article is hardened in the press mold.

[0011] The blank may be made with one or more through-passages which may have any desired shape. Examples include round or oval shaped depressions.

[0012] An essential feature of the present invention is the fact that the heat-treatment process is only accompanied by a calibration of the through-passage to final size. This ensures, that the through-passage can serve as reference point with required accuracy as reference point for subsequent manufacturing processes to which the sheet metal article may be subjected. As a consequence, the tool is exposed to less wear and the manufacturing costs are reduced. The method according to the present invention is easy to implement, cost-effective and efficient. Through calibration in the press mold, the blank can be accurately aligned in the press mold. Thus, a sheet metal article can be pressed at high accuracy in size. This is advantageous, especially for subsequent manufacturing processes.

[0013] The through-passages may be used as reference points in subsequent operations so that the sheet metal article can be precisely positioned. Of course, the through-passages may also be used for reinforcement of the sheet metal article or as assembly holes.

[0014] According to another feature of the present invention, the blank is made of steel which may have the following composition, by weight percent: 0.19 to 0.25% carbon (C), 0.15 to 0.50% silicon (Si), 1.10 to 1.40% manganese (Mn), 0.020 to 0.050% titanium (Ti), 0.002 to 0.005% boron (B), 0.02 to 0.06% aluminum (Al), a maximum of 0.025% phosphorus (P), a maximum of 0.015% sulfur (S), a maximum of 0.35% chromium (Cr), and a maximum of 0.35% molybdenum (Mo), and the balance being iron (Fe) and other incidental impurities as a result of melting.

[0015] The sheet metal blank, formed with one or more through-passages, is heated in a heat treatment apparatus to a hardening temperature, which is above Ac₃ where the steel is in austenitic state. Normally, the steel is heated to a temperature between 775° C. and 1000° C. The subsequent forming process is executed in the press mold, with the cooling operation triggering a hardening of the product to obtain a fine-grain martensitic or bainitic structure. During the hardening process, the sheet metal article is restrained in the press mold. The cooling operation may be carried out directly or indirectly. Direct cooling involves a direct contact of the sheet metal article with a coolant, whereas indirect cooling involves a cooling of the press mold or components thereof.

[0016] According to another feature of the present invention, the through-passage may be formed with a circumferential collar or with a discontinuous collar.

BRIEF DESCRIPTION OF THE DRAWING

[0017] Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

[0018]FIG. 1 is a cutaway side view of a pre-holed sheet steel blank;

[0019]FIG. 2 is a side view of the blank of FIG. 1 with formed through-passage;

[0020]FIG. 3 is a schematic sectional view of a press mold with inserted blank of FIG. 2; and

[0021]FIG. 4 is a side view of the blank after calibration of the through-passage to final shape.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals.

[0023] Turning now to the drawing, and in particular to FIG. 1, there is shown a cutaway side view of blank 1 which is pre-made with holes and made of sheet steel. The blank 1 is provided with a recess 2 having edges 3 which are formed with collars 4 in a subsequent shaping process, thereby providing the blank 1 with a through-passage 5. The through-passage 5 shown here has a circumferential collar 4. Depending on requirements at hand, it is, of course, also possible to produce a discontinuous collar 4. Basically, it is also possible to perforate the blank 1 and to shape the through-passage 5 is a single manufacturing step.

[0024] The blank 1 provided with the through-passage 5 is heated to a hardening temperature, which is above Ac₃. Depending on the steel used, the hardening temperature is between 775° C. and 1000° C. so that the steel is in an austenitic state. Subsequently, the blank 1 is shaped to size in a press mold 6 to a desired final sheet metal article 7. While still being clamped in the press mold 6, the sheet metal article 7 is hardened.

[0025] Shaping in the press mold 6 is accompanied by a calibration of the through-passage 5 to final shape. FIG. 3 shows a schematic fragmentary view of a press mold 6 involved here. The press mold 6 has an upper die 8 and a lower die 9. Accommodated in the lower die 6 is a receptacle 10 for positioning the through-passage 5. The upper die 8 has a male mold or plunger 11, which has a contour suited to the lower die 9 and cavity 10. During press stroke, the blank 1 is shaped to the sheet metal article 7 and at the same time, the through-passage 5 is calibrated to a precise final size. While still being clamped in the press mold 6, the sheet metal article 7 is hardened through rapid cooling.

[0026]FIG. 4 shows the sheet metal article 7 after calibration of the through-passage 5. In the nonlimiting example shown here, the through-passage 5 has a diameter d which has been enlarged by 3 mm, whereas the collar 4 has an inner radius r which has been reduced by 2 mm.

[0027] The thus-produced through-passage 5 may be used as reference point for accurately positioning the sheet metal article 7 in subsequent manufacturing processes. Through-passages 5 may also be provided as reinforcement of the sheet metal article 7 at suitable locations. Furthermore, the through-passages 5 may also serve as mounting holes.

[0028] While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

[0029] What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and their equivalents: 

What is claimed is:
 1. A method of making a hardened sheet metal article; comprising the steps of: forming in a blank at least one through-passage with a projecting collar; hot-forming and hardening the blank in a press mold to a sheet metal article; and calibrating the through-passage to a final shape in the press mold.
 2. The method of claim 1, and further comprising the step of pre-forming the blank before the forming step.
 3. The method of claim 2, wherein the blank is pre-made with at least one hole.
 4. The method of claim 1, wherein the through-passage is formed with a circumferential collar.
 5. The method of claim 1, wherein the through-passage is formed with a discontinuous collar.
 6. The method of claim 1, wherein the blank is made of steel having a following composition, by weight percent: 0.19 to 0.25% carbon (C), 0.15 to 0.50% silicon (Si), 1.10 to 1.40% manganese (Mn), 0.020 to 0.050% titanium (Ti), 0.002 to 0.005% boron (B), 0.02 to 0.06% aluminum (Al), a maximum of 0.025% phosphorus (P), a maximum of 0.015% sulfur (S), a maximum of 0.35% chromium (Cr), and a maximum of 0.35% molybdenum (Mo), and the balance being iron (Fe) and other incidental impurities.
 7. A method of making a hardened sheet metal article; comprising the steps of: forming in a blank at least one through-passage with a projecting collar; heating the blank; hot-forming the blank in a press mold to a sheet metal article; hardening the sheet metal article in the press mold; and calibrating the through-passage to a final shape in the press mold. 